An LED lamp known as the optical semiconductor device has a structure in which an LED die-bonded to a support substrate is sealed with a sealing material composed of transparent resin. As the sealing material, a composition mainly containing an epoxy resin has been conventionally used.
However, in use of an epoxy resin-based sealing material, due to an increase in heat generation and reduction of the wavelength of light in accordance with the recent increase in luminance of LED, cracking and yellowing have been likely to occur to cause a decrease in reliability. Hence, a composition containing silicone (polyorganosiloxane) as a main component is increasingly used because of its excellent heat resistance. In particular, an addition reaction type silicone composition is excellent in productivity because it cures in a short time by heating, and is thus suitable as the sealing material for LED. Further, a silicone cured product with high hardness is free from surface tack and thus has advantages such as capability of preventing adhesion of debris, adhesion between LEDs and the like.
However, the silicone cured product with high hardness is more fragile than the epoxy resin and generates cracks or peelings from the substrate during curing or in a reliability test of moisture absorption reflow.
As an adhesive, a potting agent, a protective coating agent, or an underfiller for the semiconductor element, there is proposed a curing silicone composition containing: (A) a solvent-soluble organopolysiloxane obtained by subjecting an organopolysiloxane having at least two alkenyl groups per molecule represented by an average composition formula: R1aSiO(4-a)/2 (where R1 represents a monovalent hydrocarbon group and 0.1 to 40 mol % of all of R1s are alkenyl groups, a is a positive number satisfying 1≦a<2) to a hydrosilylation reaction, with diorganopolysiloxane represented by a general formula: HR22Si(R22SiO)nR22SiH (where R2 represents a monovalent hydrocarbon group containing no aliphatic unsaturated bond, n is an integer of 0 to 1,000); (B) an organohydrogenpolysiloxane represented by an average composition formula: R2bHcSiO(4-b-c)/2 (where R2 is as described above, b and c are positive numbers satisfying 0.7≦b≦2.1, 0.001≦c≦1.0, and 0.8≦b+c≦2.6); and (C) a hydrosilylation reaction catalyst (refer to JP-A 2009-242627 (KOKAI), for example).
However, a cured product obtained from the silicon composition described in JP-A 2009-242627 (KOKAI) is not so large in decrease in modulus due to heating, and thus cannot sufficiently relax the stress caused by thermal expansion of the cured product itself and has difficulty in preventing generation of cracks. Further, the solvent-soluble organopolysiloxane being the (A) component is prepared by the hydrosilylation reaction using a catalyst of the same kind as the hydrosilylation reaction catalyst being the (C) component and thus has a disadvantage that the stability after mixed with the (B) component degrades because of a remnant of the catalyst used for the preparation of the (A) component.